The present invention relates to an improvement of an ion trap mass spectrometer and spectrometry in which certain specific ionic species are captured stably and then a supplementary AC electric field is generated so that the specific ionic species are excited resonantly to thereby dissociate the specific ionic species so that the dissociated ions generated at this time are subjected to the tandem mass spectrometry (MS/MS).
The ion trap mass spectrometer includes ion trap electrodes as disclosed in JP-A-9-5298. In the ion trap mass spectrometer, all of ionic species having a mass-to-charge ratio (m/Z) within a certain range are once captured stably and ions are oscillated at different frequencies in accordance with the mass-to-charge ratio thereof. This oscillation is utilized to superpose a supplementary AC electric field having a specific frequency upon a space between ion trap electrodes so that ions resonating with the supplementary AC electric field are ejected from the space to separate mass thereof. The mass spectrometry includes a method in which the mass number of substance in a sample is analyzed and a tandem mass spectrometry (MS/MS) in which ions having a certain specific mass number are dissociated as disclosed in the above publication and the dissociated ions generated at this time are subjected to the mass spectrometry. In the latter method, the supplementary AC electric field for exciting the oscillating motion of ions having the certain specific mass number resonantly is applied to the space between the ion trap electrodes in superposition manner thereupon to collide with neutral gas existing in the space to thereby dissociate the specific ionic species. In order to generate the supplementary AC electric field, a supplementary AC voltage, that is, CID (collision-induced dissociation) voltage is applied across the ion trap electrodes. The dissociated ions generated in this manner are subjected to successive scanning and analysis processing for mass separation, so that more detailed information concerning the molecular structure of the specific ions can be obtained. Accordingly, the MS/MS mass spectrometric function is one of the most important functions of the mass spectrometer in recent years.
U.S. Pat. No. 6,124,591 discloses an application method of the CID voltage in which an amplitude of the CID voltage is increased in proportion to the mass number of parent ions to be dissociated and its application time is uniformly set to be 30 ms.
The ion trap mass spectrometer includes, as shown in FIG. 2, a ring electrode 10 and two end cap electrodes 11 and 12 disposed opposite to each other so that the ring electrode is disposed between the two end cap electrodes. The ring electrode 10 and the end caps 11 and 12 are hereinafter named ion trap electrodes generically. A DC voltage U and a radio-frequency voltage V cos xcfx89t are applied across the respective electrodes to form a quadrupole electric field in the space between the electrodes. The stability of the orbit of ions captured in the field is determined by the following equation in accordance with a and q values given by a size of the apparatus (an inner diameter r0 of the ring electrode), the DC voltage U and an amplitude V and an angular frequency xcexa9 of the radio-frequency voltage applied to the electrodes and a mass-to-charge ratio m/Z.                     a        =                                                                              8                  ⁢                                      xe2x80x83                                    ⁢                  e                  ⁢                                      xe2x80x83                                    ⁢                  U                                                                      γ                    0                    2                                    ⁢                                      Ω                    2                                                              ·                              Z                m                                      ⁢                          xe2x80x83                        ⁢            q                    =                                                    4                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                V                                                              γ                  0                  2                                ⁢                                  Ω                  2                                                      ·                          Z              m                                                          (        1        )            
where Z represents ion valency number, m represent mass and e represents elementary charge. FIG. 3 is a diagram showing a stable area of the values a and q for determining a stable orbit in the space between the ion trap electrodes. Generally, since only radio-frequency voltage V cos xcexa9t (RF drive voltage) is applied to the ring electrode, all of ions positioned on the straight line of a=0 in the stable area oscillate in the space between the electrodes stably and are captured between the electrodes. At this time, the ions are arranged at different points (0, q) in the stable area shown in FIG. 3 in accordance with the mass-to-charge ratio thereof and disposed at positions from q=0 to q=0.908 on the a-axis in descending order of the mass-to-charge ratio on the basis of the above equation. Accordingly, in the ion trap mass spectrometer, all of ionic species having the mass-to-charge ratio (m/Z) within a certain range are once captured stably and at this time the ions oscillate at different frequencies in accordance with the mass-to-charge ratio (m/Z) thereof. This principle can be utilized to superpose a supplementary AC electric field having a certain specific frequency upon the space between the ion trap electrodes, so that ions resonating with the supplementary AC electric field are ejected from the space between ion trap electrodes to separate mass thereof. In the case of the tandem mass spectrometry (MS/MS), the supplementary AC electric field for exciting the oscillating motion of ions having a certain specific mass number resonantly is applied to the space between the ion trap electrodes in superposition manner thereupon to collide with neutral gas existing in the space to thereby dissociate the specific ionic species. However, the supplementary AC voltage, that is, CID voltage applied across the ion trap electrodes in order to generate the supplementary AC electric field is set to a magnitude of the degree that parent ions having a certain specific mass number are not ejected from the space between the ion trap electrodes. The dissociated ions generated in this manner are subjected to successive scanning (mass analysis scanning) for mass separation to analyze the dissociated ionic species. In order to analyze the dissociated ions with high sensitivity, high-efficient dissociation of parent ions is required.
In the above-mentioned U.S. Pat. No. 6,124,591, the amplitude of the CID voltage is increased in proportion to the mass number of parent ions to be dissociated, while its application time is uniformly set to 30 ms. Generally, ions having the high mass number or ions having very stable structure are difficult to dissociate by the collision-induced dissociation (CID). Accordingly, the ions having the higher mass number or ions having the stable structure have low dissociation efficiency, so that the result of the MS/MS spectrometry has low sensitivity. Alternatively, it is necessary to repeat a lot of spectrometry operations until a desired sensitivity is attained and the total spectrometric time is made long.
It is an object of the present invention to provide an ion trap mass spectrometer and spectrometry or spectrometric method capable of performing MS/MS spectrometry in a relatively short time with high sensitivity.
According to the present invention, a time for applying the CID voltage is adjusted in accordance with, for example, the mass number or characteristics of parent ions to be dissociated.
Thus, there can be provided the ion trap mass spectrometer and spectrometry capable of optimizing a time for applying the CID voltage in accordance with the mass number or characteristics of parent ions to be dissociated and performing MS/MS spectrometry with desired sensitivity without useless time.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.